1. Field of the Invention
The present invention relates to a method of recovering useful rare-earth elements such as Sc, Y, and lanthanoids, by using, as a raw material, a solid residue produced as a by-product in a Bayer process for producing alumina from bauxite (the solid residue is hereinafter referred to as “bauxite residue”, and in the case of containing Fe2O3 as a main component, is generally called “red mud” because of having a red color), and more particularly, to a method involving causing the rare-earth elements to leach from the bauxite residue and efficiently recovering the rare-earth elements from the resultant leachate.
2. Description of the Related Art
Rare-earth elements are widely used in applications such as a high strength Al alloy, a phosphor, a magnetic substance, optical glass, and a catalyst. In particular, the use of the rare-earth elements as materials for producing a permanent magnet has been rapidly expanding.
The rare-earth elements are also contained in bauxite, which is an ore resource of aluminum. It is known that the rare-earth elements are caused to dissolve from bauxite and are then separated and recovered. It is also known that a bauxite residue produced as a by-product in a Bayer process in producing aluminum oxide from bauxite by a Bayer method is used as a raw material, the rare-earth elements are caused to leach from the bauxite residue with sulfurous acid, and the rare-earth elements are separated and recovered from the resultant leachate (U.S. Pat. No. 5,030,424).
However, when U.S. Pat. No. 5,030,424, which relates to a method of recovering rare-earth elements from a bauxite residue, is considered, in the leaching step of causing rare-earth elements to leach from a bauxite residue into a liquid leaching agent to obtain a leachate, it is required to repeat a leaching operation two or three times, the use amount of the liquid leaching agent necessary for the operation increases, the number of times of a solid-liquid separation operation increases, and hence there is a problem in that high cost is required for recovering rare-earth elements from a bauxite residue.
In view of the foregoing, the inventors of the present invention previously proposed, as a method of solving the problem of U.S. Pat. No. 5,030,424, a method of recovering rare-earth elements from a bauxite residue, the method involving preparing a slurry having a liquid-solid ratio of from 2 to 30 and a pH of from 0.5 to 2.2 by using a bauxite residue having a specific surface area of 35 m2/g or more and by using, as a liquid leaching agent, an aqueous solution of at least one kind of mineral acid selected from sulfuric acid, hydrochloric acid, nitric acid, and sulfurous acid, subjecting the slurry to leaching treatment of rare-earth elements under a temperature condition of from room temperature to 160° C., subjecting the slurry after the leaching treatment to solid-liquid separation, and separating and recovering the rare-earth elements from the resultant leachate (International Patent WO2013/145455A).
Further, the inventors of the present invention proposed, as a method involving causing Nd and Dy, which are particularly highly beneficial as materials for, for example, a Nd—Fe—B-based permanent magnet, to leach from a bauxite residue or the like efficiently, and recovering the Nd and Dy, a method of recovering rare-earth elements, the method involving, in a leaching step, using, as a raw material, a bauxite residue containing Ca at a ratio of from 4 to 15 mass % in terms of CaO and Ti at a ratio of from 2 to 13 mass % in terms of TiO2 in a solid component (S) obtained by drying under a drying condition of 110° C. and 2 hours, using, as a liquid leaching agent, an acid aqueous solution containing hydrochloric acid and/or nitric acid and having a pH of from 0 to 2.7, and causing rare-earth elements to leach together with Ca under a heating and pressurizing condition of a temperature of from 160 to 300° C. and a pressure of from 0.65 to 10 MPa (International Patent WO2013/085052A).
Incidentally, the bauxite residue generally contains particularly large amounts of Fe2O3 and Al2O3, and even after the bauxite residue is subjected to leaching treatment by using, for example, an aqueous solution of a mineral acid, the Fe fraction and Al fraction are caused to leach as impurities in the resultant leachate. It is known that such impurities as the Fe fraction and Al fraction in the leachate form a phase of an emulsion or a suspension (hereinafter sometimes referred to as “emulsion”), for example, between an organic phase and an aqueous phase (the formed phase is hereinafter sometimes referred to as “third phase”) in a subsequent recovery step of rare-earth elements (crude recovered product) from the leachate by a solvent extraction method. When the third phase is generated, the third phase can be removed by such a method as filtration, but parts of rare-earth elements may be contained in the third phase, and hence it has been concerned that the recovery ratio of rare-earth elements may lower. It has also been concerned that, when a large amount of the leachate is treated as in an actual operation, there is a problem in that separation treatment itself of the third phase requires much cost and effort.
Thus, in each of the above-mentioned methods of recovering rare-earth elements disclosed in International Patent WO2013/145455A and International Patent WO2013/085052A, for the purpose of preventing the generation of the third phase, a pH adjuster was used to adjust the pH of the leachate to from 2.5 to 3.5 preliminarily, an Fe fraction, an Al fraction, and the like were caused to precipitate, followed by removal of the resultant precipitate, and a solvent extraction method was applied to the leachate without further treatment or the pH of the leachate was re-adjusted to from 1.2 to 2.5 and then treatment of the resultant leachate was performed by a solvent extraction method. In general, however, the resultant leachate has a pH of 1.2 or less, and it is necessary to use an extremely large amount of the pH adjuster to adjust the pH to from 2.5 to 3.5, causing cost increase. When cost calculation is performed in consideration of an actual operation level, for example, the equivalent amount of an alkali to one third of the amount of an acid necessary to adjust the pH of a raw material slurry to 1.0 is first used in a leaching step in order to increase the pH of a leachate from 1.0 to 3.0, and an acid in the half amount of the amount of the alkali is then further used to adjust the pH of the leachate from 3.0 to 1.75. Herein, when the prices of the acid and alkali are assumed to be the same, the liquid-solid ratio (L/S) thereof is set to 6, the acid is used in the amount of about 25 kg/m3 in terms of concentrated sulfuric acid, and the cost of concentrated sulfuric acid is assumed to be 10 yen/kg, a chemical cost of 250 yen/m3 is needed. As a result, the chemical cost of the above-mentioned treatment increases by 1.5 times rather than otherwise. In addition, a solid-liquid separation step of filtering fine precipitates needs to be separately performed, thus further increasing the cost of each of the methods of recovering rare-earth elements.
Thus, using such conventional pH adjusting methods requires much cost and effort. Further, examination by the inventors of the present invention has found that rare-earth elements are mixed in the precipitate deposited by the pH adjustment at as many a ratio as about 10% of the amount of all the rare-earth elements caused to leach in a leachate. Moreover, the mixed rare-earth elements contain a causative substance causing the third phase to be generated, and hence a leaching step needs to be separately prepared to recover the rare-earth elements, necessitating an extremely high cost. If the rare-earth elements in the precipitate are not recovered, there has been a problem in that the recovery ratio of rare-earth elements remarkably reduces.
On the other hand, among the impurities contained in the leachate, such as an Fe fraction and an Al fraction, the amount of the Al fraction is, in particular, remarkably larger than the total amount of rare-earth elements caused to leach in the leachate. It is known that the Al fraction shows the same behavior as light rare-earth elements in a solvent extraction method (extraction step) using a phosphoric acid ester-based extractant, and hence, when such a method as being able to separate an Al fraction from rare-earth elements like the pH adjustment described above is not performed, there has been a problem in that it is extremely difficult to remove an Al fraction in a recovery step (extraction step, back extraction step, or the like) of rare-earth elements (crude recovered product) from a leachate by a solvent extraction method. Thus, it has been desired to develop a method of recovering rare-earth elements free of an Al fraction without performing, for example, the pH adjustment described above.
In addition, the impurities caused to leach in the leachate include high solvent-extractable metal impurities such as Fe, Ti, Th, Zr, and U, low solvent-extractable metal impurities such as Ca, Mn, Si, and Na, and organic substances. In this connection, the high solvent-extractable metal impurities have a poor back extraction characteristic. Thus, although back extraction thereof is possible to a certain extent in the subsequent back extraction step, when continuous treatment is performed, parts of the high solvent-extractable metal impurities are not back-extracted in a back extractant but remain in the leachate, and are gradually accumulated in an extractant (extraction treatment liquid containing a phosphoric acid ester-based extractant or the like) used in the solvent extraction step, and it has been confirmed that there is a problem in that, when the accumulation concentration thereof exceeds the accumulation limit concentration thereof, the extraction efficiency of rare-earth elements in the extraction treatment liquid is inhibited. On the other hand, the low solvent-extractable metal impurities are not easily extracted by the solvent extraction method (extraction step), thus not particularly causing any problem. However, when sulfuric acid is used in a leaching step, plaster precipitates in the sulfuric acid, and it has been confirmed that there is a problem in that, when the metal impurities are contained at a high concentration, the metal impurities and rare-earth elements coprecipitate, causing recovery loss of rare-earth elements. In addition, it has been confirmed that, when such a conventional method as significant pH adjustment by which the pH of a leachate is adjusted to about 3 is not performed, a third phase is generated in, for example, a recovery step of rare-earth elements (crude recovered product) from the leachate by a solvent extraction method. Further, it has also been confirmed that humic organic impurities caused to leach in the leachate cause scum to occur in a settler, thereby causing an obstacle to happen in the recovery step, and rare-earth elements are contained in the humic organic impurities. It has also been confirmed that there is a problem in that the precipitates, the third phase, the scum, and the like cause the reduction of the recovery ratio of rare-earth elements.